Mounting evidences suggests that cancers such as melanoma produce programmed death protein-1 ligand 1 (PD-L1) to suppress T cell activation and immunity via interaction with PD-1 (receptor of PD-L1). Emerging immune therapies such as anti-PD1 and anti-PD-L1 treatments have shown great success in treating cancers, such as melanoma and head and neck cancers. However, the unique role of PD-L1/PD-1 in pain regulation is unknown. Our work in progress has demonstrated the presence of anatomical and functional PD-1 in primary sensory neurons and identified PD-L1 as a novel pain inhibitor, produced not only by cancer cells but also by normal neural tissues. We hypothesize that PD-L1 masks cancer pain and inhibits baseline pain and neuropathic pain via silencing PD-1 expressing nociceptive neurons. We also hypothesize that PD-L1 can inhibit somatic and trigeminal neuropathic pain via suppressing microglial and T-cell activation in the spinal cord and brain stem. Of interest recent studies have shown that microglia and T cells in the spinal cord contribute to neuropathic pain in a sex-dependent manner. Consistently, our pilot study shows that intrathecal inhibition of caspase-6 and p38 signaling inhibits nerve injury-induced mechanical allodynia in male but not female mice. We further hypothesize that PD-L1 inhibits microglial and T-cell activation in a sex-dependent manner. The overall goal of this application is to investigate how the PD-L1/PD-1 pathway can mask cancer pain and also suppress neuropathic pain via regulating the function of neurons, microglia, and T-cells. We will use mouse melanoma and neuropathic pain models of both sexes to test our hypotheses by accomplishing the following 3 specific aims: Aim 1. Test the hypothesis that the PD-L1/PD-1 pathway can mask skin cancer pain, suppress somatic and trigeminal neuropathic pain, and inhibit baseline pain; Aim 2. Test the hypothesis that activation of the PD- L1/PD-1 pathway can suppress neuronal excitability in mouse and human DRG/TG neurons and inhibit synaptic plasticity in mouse spinal cord neurons; Aim 3. Test the hypothesis that the PD-L1/PD-1 pathway can inhibit pain via suppressing microglial and T-cell activation in a sex-dependent manner. This project will identify PD-L1 as a novel endogenous pain inhibitor and provide new insights into neuron-glia, neuron-immune, and neuron- cancer interactions. Manipulation of PD-L1/PD-1 signaling may lead to the development of novel analgesics.